So-called "conventional" (i.e., non-trunked) RF repeaters and associated communications systems have provided reliable, cost-effective communications for mobile and portable RF transceiver users. As those skilled in the art know, non-trunked RF base stations repeat (or relay), on a given output frequency, signals received on a given input frequency. In such systems, user transceivers (e.g., mobile units installed in vehicles, and portable units carried on tile person) in the field transmit RF signals on the repeater's input frequency and receive RF signals on the repeater output frequency). Since the base station typically has much higher effective radiated power than the mobiles and portables, the "talk range" of each individual user is increased substantially without requiring the individual users to have high power equipment. Such "conventional" RF repeater systems provide cost-effective, reliable RF communications to many thousands of RF transceiver users.
As those skilled in this art know, the "squelch" circuit of an RF receiver acts as a gate that selectively permits audio received by the receiver to pass the receiver audio output. When the squelch is open, the signal passes; when squelch is closed, the signal is blocked. As will be explained, many such conventional base station systems provide limited call privacy, access control and "frequency sharing" features by using so-called "channel guard" CTCSS "selective squelch" subaudible audio tones to control squelch circuits throughout the communications system.
In "selective squelch" equipped RF repeater/base station systems, each user RF transceiver is provided with a subaudible tone generator and a subaudible tone decoder. A subaudible tone is continually impressed (modulated) on the carrier of each of the user transceiver transmissions for the duration of transmission (since the tones are subaudible, they occupy audio frequencies that are below those audio frequencies used for voice communication--and hence do not interfere or overlap with the voice communication signals carried by the same RF carrier).
The non-trunked base station has its own subaudible tone decoder which decodes the particular subaudible audio frequencies transmitted by the user transceivers. Upon receiving a carrier modulated by a subaudible tone at the appropriate frequency is impressed, the base station opens its receiver squelch and permits the received audio signal (including the subaudible tone) to be retransmitted (or possibly regenerated) by the repeater transmitter. On the other hand, if a signal received by the base station does not include the appropriate subaudible tone, the base station receiver squelch remains closed and the received signal is not repeated. This arrangement thus prevents all but those transceivers equipped to generate the appropriate subaudible tone from "accessing" the communications system.
Similarly, the receiver section of a user transceiver opens its squelch (permitting received audio to be amplified and passed to a loudspeaker) only when it receives an RF carrier modulated by the appropriate subaudible tone (the user transceiver typically also filters out the subaudible signals from the audio provided to the loudspeaker to prevent very low frequency sounds from being generated in response to the tones). User transceivers effectively ignore received signals that do not include the appropriate subaudible signals--thus relieving the user from continually hearing other people's conversations.
There are several different standard CTCSS tone frequencies ranging from about 60 Hz to about 250 Hz or so. These tone frequencies are standardized to facilitate equipment compatibility between different manufacturers. It is possible to have different groups of user transceivers (e.g., the road maintenance crew and the trash collection crew of a county government) share the same RF base station and associated RF channel by having the different transceiver groups operate using different CTCSS frequencies. The road maintenance crew, for example, can be provided with transceivers having tone generators and decoders that operate at a first CTCSS frequency; and the transceivers used by the trash collection crew can have tone generators and decoders that operate at a second CTCSS frequency different from the first frequency. The common base station opens squelch in response to receipt of either CTCSS frequency. In this way, the road maintenance crew is only disturbed by transmissions originating from other road maintenance crew transceivers; and similarly, the trash collection crew only hears transmissions originating from other trash collection crew transceivers. So long as the different groups comprise relatively infrequent users, the groups are able to share the same channel and associated base station without substantial interference with one another. In addition, the selective squelch arrangement provides a degree of privacy between different groups of users.
So-called "trunked" RF communications systems provide for a more efficient way of sharing RF channels among multiple user transceivers--and thus provide certain advantages over conventional non-trunked systems. For example, trunking increases channel utilization by permitting efficient time-sharing of channels by different users or groups of users. In many trunked RF systems (e.g., the "DAC Multisite" systems made by Ericsson-GE, the assignee of the subject application), user radios are temporarily "assigned" to RF working channels only while they are engaged in active communications, and monitor a digital control channel at all other times. Such exemplary digital trunking systems provide call privacy in addition to more efficient frequency utilization (since only user radios that are actively involved in a particular communique are "assigned" to the RF channel carrying that communique--and then only for the duration of the communique). "Digitally trunked" communications systems are also capable of providing a wide range of advanced features (e.g., dynamic regrouping, capability to transmit digital data over the RF channels, etc.) not provided in prior "conventional" non-trunked systems. Such capabilities make trunked systems the systems of choice for many new equipment acquisitions.
Purchasing new digitally trunked equipment to support a digitally trunked communications system is often expensive, but it can be even more expensive if all of the customer's radio equipment must be replaced in order for any of the customer's user to take advantage of digital trunking. A customer who has been operating a conventional system for awhile typically has devoted substantial resources to the equipment and training associated with his conventional system. Unfortunately, non-trunked CTCSS type user transceivers and base stations are generally incompatible with state-of-the-art digitally trunked communications systems. A customer thinking about purchasing a digitally trunked system but having an already installed conventional system is typically very worried about whether the new system will make his existing system obsolete. There is thus a great need to design digitally trunked communications systems such that existing tone-driven conventional equipment is at least partially compatible and/or can be integrated with newly purchased state-of-the-art digitally trunked RF communications systems and components.
Suppose, for example, a county government that uses a radio repeater system to support the communications of a variety of different governmental services (e.g., police, fire, paramedics and ambulances, trash removal, road maintenance, building inspectors, etc.). Most county governments have used radio repeater systems in one form or another for many years, and may have purchased (and continue to operate successfully) "conventional" (non-trunked) RF repeater ("base") stations and associated transceivers. The county may contemplate "upgrading" its RF communications system such that the advantages and features provided by digitally trunked components can be made available to its more critical services (e.g., police, fire, ambulance). However, the county may be unwilling to spend the additional money necessary to provide digitally trunked equipment for its less critical services (e.g., trash collection, building inspectors). While it is possible to maintain the two systems (digitally trunked, and conventional) side by side (providing a human interface in the form of a dispatcher between the two incompatible systems), it would be highly desirable to provide at least a limited automatic interconnection or link between the two systems such that calls handled by the digitally trunked system could also be routed to and/or from the conventional system.
Moreover, there is now a trend toward creating networks of digitally trunked repeater stations (see, for example, the earlier filed copending patent applications identified in the "Cross-Reference to Related Applications" section of this patent application). Such networks provide great advantages by coordinating RF communications across multiple RF repeater sites. It would be highly desirable to be able to integrate conventional non-trunked base stations into such network arrangements.
Unfortunately, it is difficult to provide compatibility or integration between a digitally trunked network and a conventional base station. Conventional base stations typically are controlled by analog voltage levels and/or via audio tones of specified frequencies. Such voltage and/or tone control is generally incompatible with digitally trunked switches (which may, for example, use serial data communications busses and associated protocols for controlling base stations). In addition, a digitally trunked system typically designates and/or identifies RF transceivers in the field with multi-bit digital identification numbers (e.g., in EGE's DAC system, individual radio transceivers are assigned unique digital identifiers). Such digital identification is generally incompatible with CTCSS type selective squelch systems (as described above) which use analog audio tones to specify groups of radio transceivers.
Commonly-assigned U.S. Pat. No. 5,241,537 entitled "Conventional Base Station Interface Architecture For RF Trunking Multisite Switch"; describes one attempt to provide such integration. Although the Gulliford et al. patent application has an effective filing date which is prior to the subject application, it may not be prior art against the subject invention. In any event, the Gulliford et al disclosure is referenced as being an example of one approach for providing an interface between a conventional base station and a digitally trunked RF communications system. Gulliford et al describe, in their patent application, a technique for interfacing plural conventional base stations with a multi-site RF trunking switch/network through a "CVIM". This CVIM includes, in their described preferred embodiment, a controller module and backup controller module; and plural audio modules. Special purpose messages may be directed to the CVIM in the Gulliford et al arrangement to provide certain functionality. While the Gulliford et al arrangement has been highly successful in its own right, it may not provide a cost-effective solution to certain customers who wish to interface only one (or a very small number) of conventional base stations to their digital trunking system.
Thus, a need exists for a cost-effective, relatively simple arrangement for interfacing a conventional (i.e., non-trunked) RF base station with a digitally trunked radio communications system. It would be highly desirable if such an interfacing arrangement were capable of providing interfacing for conventional base stations of various configurations (e.g., made by different manufacturers) to facilitate integration of a wide variety of existing customer equipment with state-of-the-art digitally trunked radio communications systems.
The present invention, in accordance with one of its aspects, facilitates communications and interactions between: (a) conventional RF repeater systems using subaudible tone signalling; and (b) a digitally trunked RF communications system (e.g., EGE's DAC system). The conventional network interface provided by the present invention might be regarded as a "gateway" between the digitally trunked switch and a non-trunked tone-operated system.
In accordance with one aspect of a feature provided by the present invention, an interface between a non-trunked RF base station and a digitally trunked RF communications system provides mapping between subaudible tone frequencies and transceiver digital identifiers. The interface may, for example, map a so-called "group identification" digital value provided by the digitally trunked system into a particular subaudible tone frequency produced by the conventional base station--and vice versa. Thus, it becomes possible to define a "super group" including both digitally trunked user transceivers and non-trunked, selective squelch tone controlled user transceivers--and to conveniently handle communications to/from all of the user transceivers within such "super group" automatically within the same integrated RF communications system.
Thus, the present invention may facilitate communications between radios from specific Groups within the EGE wide band and narrow band DAC systems and conventional non-trunked radios using specific subaudible frequencies.